An ideal diode is a two terminal device that has an anode electrode and a cathode electrode. If the voltage on the anode is positive with respect to the voltage on the cathode, then a forward voltage is said to be present across the diode. If the voltage on the anode is negative with respect to the voltage on the cathode, then a reverse voltage is said to be present across the diode. For a forward voltage greater than a threshold voltage, the diode is to conduct a forward current and is to have as small of a resistance to current flow as possible. For a reverse voltage, the diode is to block reverse current flow and is to present as high of a resistance to current flow as possible. In a real diode, however, when the diode is forward biased and is then switched to be reverse biased, a burst of reverse current flows from the cathode to the anode. The PN junction of the diode has a capacitance. A charge is stored in this capacitance. When the voltage across the diode is switched from being a forward voltage to being a reverse voltage, the charge stored in the junction must be discharged and this discharging gives rise to the burst of reverse current. Once the charge has been discharged, then the reverse current stops flowing and the diode is seen to block reverse current flow. The magnitude of the burst of reverse current is generally specified as the reverse recovery peak current IRM. The amount of time for the reverse recovery current to decrease from IRM to a specified reverse current (for example, 0.2 of IRM) is specified as the reverse recovery time trr.
The reverse recovery characteristic of a diode can be described as “snappy” or as “soft”. During a reverse recovery episode, once the reverse current has peaked to its largest IRM value, if the reverse current then abruptly returns to zero current then the diode may be said to have a snappy characteristic. If, on the other hand, the reverse current returns to zero current more slowly then the diode is said to have a soft characteristic.
In a typical power diode application, if the diode has a snappy reverse recovery characteristic, then the reverse recovery current may decrease to zero current so fast and in such a way that the reverse recovery current actually overshoots zero current and for a short period of time becomes a forward current. The reverse recovery current may then oscillate around zero current before it stabilizes to its zero value for steady state reverse bias operation. This overshoot and/or ringing of the reverse recovery current is accompanied by a corresponding momentary high reverse voltage and/or voltage ringing. The momentary high reverse voltage may be so large that it may exceed the rated reverse blocking voltage of the diode, thereby destroying the diode. In order to prevent the diode from being destroyed, it may be necessary to provide expensive and cumbersome snubber circuitry.